1. Field of the Invention
This invention relates to vehicle occupant restraint systems and more particularly to a seat belt pretensioner for taking up slack in a seat belt in the event of a vehicle collision.
2. Discussion of Prior Art
A typical inertia reel seat belt comprises a reel of webbing fitted at one end with a locking device that only functions if the reel spins too quickly. During a collision, standard inertia reel seat belts will typically allow 120 to 200 mm of slack before the locking mechanism functions. Devices which are more sensitive than this are impractical when the user unreels the belt to lock it into position in normal use. The delay in operation can allow the occupant to move out of the seat before being arrested and whilst this may avoid a fatality, it can lead to serious whiplash injuries.
Seat belt pretensioners are now widely used in vehicles to provide additional protection to the occupants in the event of a collision. They are normally connected to collision-sensing devices and use stored energy to wind in sufficient slack in the belt to lock the belt before the user can begin to move. The energy can be stored in the form of a spring or in a pyrotechnic gas-generating composition.
In a typical application, the function of the seat belt pretensioner is to rotate the seat belt retractor or take-up reel rapidly in a belt retraction direction to pull the belt against the vehicle occupant and eliminate slack in the belt. Many such pretensioner devices have been proposed, and exemplary of such prior art constructions are those disclosed in U.S. Pat. Nos. 4,178,016, 4,789,185 and 5,145,209; European patent number 0480137; German patent application number DE3715845 and UK patent application number 2258603. The entire disclosures of these prior art patents are incorporated herein by reference.
The technical requirements for a seat belt pretensioner are very numerous. Firstly it must not interfere with the normal operation of the inertia reel seat belt retractor. Secondly it must be capable of activation within a few milliseconds in order to provide adequate protection to the vehicle occupant in the event of a collision. Thirdly, vehicle manufactures' specifications now require that the pretensioner should exert a force of 700 to 800 Newtons and retract the seat belt by a minimum of 100 mm and a maximum of 200 mm. Fourthly, after activation of the seat belt pretensioner, it should preferably not interfere with the normal operation of the inertia reel seat belt retractor, in the event that the vehicle can still be driven after the collision incident.
Further very important practical requirements are that the seat belt pretensioner should be highly compact, fit on to existing inertia reel seat belt systems and preferably fit the housing profile of existing equipment.
Seat belt pretensioners which have been proposed hitherto either do not meet the technical requirements outlined above, or are unacceptably bulky or impractical.